The present invention relates to a motor-driven proportional fluid flow control valve, and more particularly to a motor-driven proportional fluid flow control valve having an electromagnetic coil supplied with a constant current for closing the valve with a slide valve body spaced from a valve stop surface.
Motor-driven proportional fluid flow control valves have an electromagnetically operated linear motor for opening and closing a slide valve body to allow a fluid to flow at a rate substantially proportional to a current applied to the linear motor. Such motor-driven proportional fluid flow control valves have found wide use in automobile emission control devices. In such an application, the valve has an inlet port connected to an air cleaner and an outlet port connected to a carburetor and an air bleed port. The current supplied to the valve is varied dependent on signals representative of the vacuum, temperature and rpm of the engine for controlling the amount of air flowing through the air bleed port to achieve a carburetor air-fuel ratio required for an exhaust gas density optimum for a three-way catalyst used.
When the motor-driven proportional fluid flow control valve is closed, the slide valve body is urged by a spring to be held at its end against a stop surface of a valve casing. This is disadvantageous in that the slide valve body tends to hit the stop surface repeatedly and produce striking sounds, owing to the duty frequency of a small current which flows through the electromagnetic coil, as well as the vibration thereof, when a drive circuit starts operating. One solution to this problem has been to use a damper such as of rubber for suppressing such striking sounds. However, the rubber damper has proven unsatisfactory in that it has a limited service life.